R.O.C. patent publication No. 291130 discloses a “Power supply power factor correction apparatus” which provides a circuit design to solve “resonant wave distortion” occurred to the first inductor and the second inductor on a conventional power factor correction circuit when the power conversion circuit is in different duty conditions. It proposes a power supply selection device which has nine terminals. The power supply selection device consists of two ON-OFF switches and a switch. One of the two ON-OFF switches aims to select the duty condition (input power voltage of 110V or 220V) of the power conversion circuit, while another ON-OFF switch aims to open or close the circuit. The switch is to make two inductors of the power factor correction circuit in a parallel coupling condition or a serial coupling condition when the power conversion circuit is in different duty conditions. Thereby when input power supply is in a high voltage duty condition, the inductors are coupled in series to increase the inductance and improve the problem of “resonant wave distortion”, and when the input power supply is in a low voltage duty condition, the inductors are coupled in parallel to divide the larger current and protect the circuit.
However, the aforesaid technique does not cover the conventional six-terminal switches, and nine-terminal switches must be used. To switch parallel or serial coupling of the inductors, a separated nine-terminal switch has to be fabricated. The production and design costs are higher than the six-terminal switch. Moreover, the nine-terminal switch has a bigger size than the six-terminal switch. To allocate more space on the general power supply panel to install such type of switch is difficult. Hence the applicability of that technique is limited in the present industry condition. It is not widely accepted.